This underlying problem of collective action is central to the eld of environmental planning, where participatory exercises potentially affect large sections of the population. Attempts to control urban air pollution would appear to represent the paradigmatic example. The bene ts of a strategy to improve air quality fall on all residents of a city, irrespective of their participation in the formulation of the strategy, so there is little incentive to participate. Neither is this incentive to free-ride diminished by the threat of collective sanction, because the size of the urban population means that shirking is almost anonymous and the costs of monitoring are prohibitive (Rydin, 1998). Therefore, in so far as members of the public spontaneously and actively participate in the environmen-tal policy process, this participation is likely to be concentrated around issues of nimbyism. Here, the localised nature of nimbyist disputes and the smaller number of affected parties raise the likelihood of individual inuence and lower the cost of monitoring free-riders.